Vascular smooth muscle is derived from multiple embryonic origins. Our laboratory has utilized murine models to define the population of arterial smooth muscle that is derived from neural crest. This group of cells is uniquely affected in common forms of congenital heart disease including those affecting the cardiac outflow tract and vessel segment derived from aortic arch arteries. We have developed genetic tools to follow the migration and differentiation of neural crest derived smooth muscle in normal and mutant backgrounds. Emerging data indicates that these processes are regulated by gene expression within neural crest cells and also by secreted factors emanating from neighboring tissues. Thus, mutations in Tbx1, a gene implicated in the congentital cardiac defects associated with DiGeorge syndrome, cause a deficiency of neural crest-derived smooth muscle differentiation, though Tbx1 itself is not expressed in neural crest cells. Rather, it is expressed in a number of tissues that are in close proximity to migrating and maturing neural crest cells as they encase the aortic arch arteries. These tissues include pharyngeal endoderm, pharyngeal mesenchyme, head mesenchyme and cardiac mesoderm. In some or all of these tissues, Tbx1 can be regulated by sonic hedgehog (Shh) and can induce expression of members of the Fibroblast growth factor (Fgf) family. We hypothesize that Fgfs induced by Tbx1 signal to neural crest cells and induce smooth muscle differentiation. We propose experiments to determine the tissues in which Tbxl expression is required for vascular development and to elucidate downstream signaling pathways, including those mediated by specific Fgfs, required for smooth muscle differentiation. Specific aims include the analysis of Tbx1 enhancer elements and the creation of Cre-expressing mice utilizing these enhancers, analysis of Shh regulation of Tbx1, characterization of Tbx1 transcriptional activity including DNA binding and interaction with a novel co-regulator (Ash2I), examination of the ability of Tbx1 to regulate Fgf10 and Fgf8, and genetic manipulation of Fgf receptors on neural crest cells. Finally, we will examine the role of the transcriptional co-activator myocardin in neural crest-derived smooth muscle differentiation. Thus, we hope to elucidate a molecular pathway by which specific neighboring cells induce gene expression and smooth muscle differentiation in neural crest.